Answer :

D3xt3R
It happens when you have 2 angles and 1 side in common at the same spot in two triangle at same time...

see the image...

Side

[tex]AC\equiv DF[/tex]

First Angle

[tex]A\^BC\equiv D\^EF[/tex]

Second Angle

[tex]B\^CA\equiv E\^FD[/tex]